Shape memory alloy (SMA) is an attractive actuator material to use in microelectromechanical systems (MEMS) when high forces and work are needed. SMA outperforms most other actuation principles at the microscale by more than an order of magnitude because of the high work density that SMAs offer, see M. Kohl et al.: “Shape memory microactuators”, Springer, pp. 23-24, (2004).
Traditionally there are mainly two ways of integrating SMA materials into microsystems. The first approach is a pick and place approach where the SMA material and the microsystem is manufactured separately and then combined in a subsequent step, see K. Skrobanek et al.: “Stress-optimized shape memory microvalves”, MEMS Proc., pp. 256-261, (1997). This approach has the advantage of allowing integration of bulk SMA materials, which are commercially available and offered in a wide thickness range at comparably low material cost. However, the SMA integration is performed on a per-device level which results in high assembly cost. The second approach is based on sputter deposition of thin NiTi films directly onto the microstructure, see P. Krulevitch et al.: “Thin film shape memory alloy microactuators”, J. Microelectromech. Syst., vol. 5, no. 4, pp. 270-82 (1996), which has the benefit of allowing wafer level processing. However, due to a difficult deposition controllability, the process is limited in reproducibility of transformation temperatures and strains and NiTi sputter deposition is mostly feasible for thicknesses of less than 10 μm, see S. Miyazaki et al.: “Development of high-speed microactuators utilizing sputterdeposited TiNi-base shape memory alloy thin films”, Actuator Proc., pp.372-377 (2008).
Wafer level integration of SMA wires onto silicon microstuctures, with the benefit of both wafer level integration and the use of bulk SMA materials, has been shown for microactuators with excellent performance, see D. Clausi et al.: “Design and wafer-level fabrication of SMA wire microactuators on silicon”, JMEMS, vol. 19, no. 4 (2010). However, no standardized fabrication process has yet been established or suggested. The placement of the wires requires specially designed tools with manual wire handling, alignment and integration. In contrast, wire bonding is an extremely mature, cost-effective and broadly available backend process for electrical interconnects, see W. J. Greig et al.: “Integrated circuit packaging, assembly and interconnections”, Springer, pp.103-128, (2007). It is very attractive to utilize this standard technology due to its very good availability and high performance in terms of reliability, throughput and placement accuracy, with speeds up to 22 bonds per second and placement accuracies within 2 μm. However, direct wire bonding of NiTi SMA wires is not feasible due to the Vickers hardness of the NiTi material, which is one order of magnitude higher, see K. Gall et al.: “Instrumented micro-indentation of NiTi shape-memory alloys”, Acta Materialia, vol. 49, no. 16, pp. 3205-3217 (2001), as compared to common wire bonding materials such as gold and aluminum.